These are the secret achievements we know for now...

Stamp Of Approval - 5 gamerpoints - You created your own band logo.
One Of A Kind Axe - 5 gamerpoints - You created your own custom guitar.
Warrior Of Rock - 5 gamerpoints - You created a male band member.
Custom Beats - 5 gamerpoints - You created your own custom drums.
Rock Maiden - 5 gamerpoints - You created a female band member.
Inked - 5 gamerpoints - You created a custom tattoo.
Leading Lady - 10 gamerpoints - You rocked out as Hayley Williams.
The Experience - 10 gamerpoints - You rocked out as Jimi Hendrix.
The Dark Prince - 10 gamerpoints - You rocked out as Ozzy Osbourne.
Pumpkin Smasher - 10 gamerpoints - You rocked out as Billy Corgan.
Bad to the Bone - 10 gamerpoints - You rocked out as the Skeleton.
Wylde Man - 10 gamerpoints - You rocked out as Zack Wylde.
Motorcity Madman - 10 gamerpoints - You rocked out as Ted Nugent.